1. Field of the Invention
This invention relates to tools used for mineral recovery from the earth. In particular, the present invention is a novel pressure relief device for lubricants sealed in rolling cutter earth boring drill bits.
2. Description of the Related Art
Modern day rolling cutter earth boring bits are typically used for drilling wells for the recovery of oil, gas and other minerals from the earth. Most rolling cutter drill bits used for deep well drilling typically have a sealed bearing system filled with a lubricant. Bits of this type have been used for many years in the oil and gas well drilling industry and are well known.
In order for sealed rolling cutter drill bits to operate effectively when drilling these deep wells, a means must be provided in the bit's lubrication system to equalize the pressure between the lubricant sealed within the bit and the drilling fluid outside. A pressure equalization device is also helpful in reducing pressure fluctuations near the dynamic seal during operation. These pressure fluctuations are caused by the piston effect of the rolling cutter as it moves axially on the bearing shaft during operation due to normal bearing clearances.
Pressure equalization devices in rolling cutter drill bits may utilize slidable pistons, bellows devices, or resilient diaphragms or membranes. The common trait of all pressure equalizing devices for drill bits is that they are able to displace a volume of lubricant in response to pressure differentials.
Exemplary pressure equalizing devices for rolling cutter drill bits are described in U.S. Pat. Nos. 3,007,751, 3,048,230, 3,230,319, 3,230,020, 3,721,306, 3,917,028, 4,055,225, 4,276,946, 4,335,791, 4,407,375, 4,552,228, 4,577,705, 4,597,455, 4,942,930, 5,490,570, and 5,558,172 all herein incorporated by reference for all they disclose.
For maximum effectiveness, it is desirable for the lubricant pressure-equalizing device to have the ability to displace a large volume of lubricant in a lubricant reservoir. However, because there is limited space available in the bodies of rolling cutter drill bits for lubricant reservoirs, the reservoir volume is often much smaller than what is desirable.
The most commonly used way to reduce the lubricant reservoir volume and still effectively control the pressure differentials between the lubricant and the drilling fluid is to utilize lubricant pressure relieving devices. A typical pressure relieving device allows a volume of lubricant to be released from the bit when a predetermined differential pressure is exceeded. A lubricant system that utilizes a relief valve can allow nearly the entire volume of the reservoir to be filled with lubricant during assembly. Should the lubricant volume expand beyond the limit of movement of the compensating device, lubricant is expelled.
A common way of accomplishing lubricant expulsion is through mechanical relief valves. Mechanical lubricant pressure relief devices for rolling cutter drill bits are shown in U.S. Pat. Nos. 3,476,195, 3,942,596, 4,019,785, 4,161,223, 4,388,984, 4,593,775, 4,865,136, all herein incorporated by reference for all they disclose. Mechanical relief valves, however, tend to become packed with fine abrasive particles from the drilling fluid. When the valves clog up, they no longer relieve the lubricant at a predictable pressure differential.
Mechanical valves are also subject to the extreme accelerations and vibrations experienced by the rolling cutter drill bit as it drills into the earth. These forces can cause the operating mechanisms in the mechanical relief valves to unseat, releasing lubricant or allowing the ingress of drilling fluid. The relieving pressures must be set high enough, typically above 150 PSI differential (PSID), to keep the valve firmly seated in the presence of these vibrations. Also, if abrasive particles have been trapped in the passageways of the valve, these particles can become lodged in the valve seats, holding the valve open, and allowing an undesirable exchange of lubricant, and drilling fluids.
Another means of accomplishing lubricant expulsion in rolling cutter drill bits utilizes the elasticity of the pressure compensation membrane, as shown in U.S. Pat. Nos. 3,847,234, 4,727,942, 4,887,675, and 5,072,795 all herein incorporated by reference for all they disclose. In these patents, an aperture is formed in a portion of the membrane. The aperture is held closed by the elasticity of the membrane material. It remains closed until the differential pressure of the lubricant stretches the membrane and opens the aperture, allowing lubricant to be released.
There are several advantages of this design over mechanical relief valves. Because the aperture is integral with the pressure compensation membrane, it is not particularly affected by bit vibrations. Therefore, this type of relief mechanism is not prone to opening during the excessive mechanical vibrations that occur in rolling cutter drill bits. The manner of operation of the relief valve in the membrane minimizes its likelihood for becoming clogged with the abrasives, and being held open by these abrasives.
A further advantage of this type pressure compensating membrane with an integral pressure relief device is that two-way pressure relief is possible. As explained in U.S. Pat. Nos. 3,847,234 and 5,072,795 (referenced above), there are times when it is advantageous for a relief device to allow drilling fluid into the bit during operation. When lubricant is lost from the bearing during operation, very high-pressure differentials may occur when the lubricant reservoir becomes depleted. These very high pressure differentials lead to rapid failure of the dynamic seal in the rolling cutter, and the consequent failure of the bit. If drilling fluid is allowed to enter, the high pressure differentials do not occur, and failure of the bit is delayed.
A disadvantage of the design shown in the '234 and '795 Patents is that very small differences in the aperture geometry cause large changes in the relief pressures. Since the aperture of the '234 and '795 Patents is traditionally made by forming a slit in the membrane with a puncture tool, factors such as tool wear, placement of the parts while puncturing, and direction of puncture all affect relief pressure. Pressure variations of +/-50 PSI are common. Consequently, the average relief pressure must be set to a fairly high value of about 125 PSID or more.
Even when the manufacturing variations are carefully controlled, and the relief pressures are set lower than about 80 PSID, the lubricant tends to `bleed` or leak slowly through the aperture. It is believed that at the lower relief pressure settings, there is not enough elastic closure force on the aperture to prevent small amounts of lubricant and/or drilling fluids from entering the aperture. When the membrane moves in response to a differential pressure, the lubricant trapped in the aperture is squeezed out, resulting in, effectively, a slow leak.
In rolling cutter drill bits utilizing volume compensating rigid face seals, such as shown in U.S. Pat. No. 5,875,861 herein incorporated by reference for all it discloses, it is desirable to reduce the lubricant relief setting to a very low value, typically less than about 50 PSID. Reducing the relief pressure helps prevent the seal assembly from losing its ability for volume compensation.
The lubricant pressure relief devices described above work very well with packing ring type seals, and with self-relieving face type seals. Unfortunately, as stated above existing pressure relief devices typically need to be set above 125 PSID for reliable operation.
Many other types of down hole tools such as down hole motors, MWD tools, powered wellbore deviation tools, etc., also have sealed lubricants with pressure compensation and pressure relief devices. Any of these devices may also potentially benefit from a reliable low differential lubricant pressure relief valve.